Filters that have a complex structure normal to the optical axis play an important part in many fields of optics. For example, so-called barrier filters are used to enable the viewer of a display screen to have a three-dimensional vision without further aids, provided that the picture contents to be displayed on the screen have been processed accordingly. Such filter structures can consist, for example, of an array, normal to the optical axis, of areas that are opaque and such that are transparent to light in the visible wavelength range, i.e. in a wavelength range from about 400 nm to 800 nm, arranged in a specified complex pattern. Areas that are transparent only to specified wavelengths or wavelength ranges are also feasible, just as are grid structures, although the latter are less used in connection with the implementation of three dimensional presentations.
These filter structures are commonly applied onto a substrate transparent to light. Glass or a suitable plastic are examples of eligible substrate materials.
First, the substrate is laminated with a cold laminating film on top. For this purpose, the cold laminating sheet is provided with an adhesive coating on its bottom side, which is joined with the glass substrate. The top side of the cold laminating sheet also has such an adhesive coating. On the latter, a photographic film is laminated as a next step. Alternatively one can do without the carrier sheet, using a single adhesive coating only.
Such a photographic film comprises, as a rule, a base film made, for example, of cellulose or polyester, and a thin layer that is at first sensitive to light, known as photographic emulsion layer, which is actually a suspension, i.e. a mixture of a liquid and solid matter dispersed in it, to be precise, a slurry of crystals finely dispersed in gelatin. Usually, the crystals are some silver halide, i.e. silver chloride, silver bromide or silver iodide. The photographic emulsion layer is then exposed to light so that a pattern corresponding to the complex optical filter structure is imaged on the film. This can be done, for example, with the aid of a prefabricated mask. Subsequently, the photographic filter is developed and fixed—as is common and long well-known in prior art—to make the optical filter structure impressed into the photographic emulsion layer permanently light-fast.
In the next step, the side of the film carrying the photographic emulsion layer is laminated to the adhesive coating of the cold laminating sheet. Thus one obtains a system of layers of glass-adhesive-cold laminating sheet-adhesive-photographic emulsion layer-base film.
The two laminating operations generally involve the risk that dust particles, fluff or other flaws such as, for example, air bubbles, get embedded into the photographic emulsion layer, which may lead to noticeable impairments of the optical action of the filter structure. In the worst case, this makes the filter unserviceable. Especially in case of large-area photographic films, the pressure of between 50 and 70 kPa that has to be applied for laminating, commonly by means of a roller or a cylinder, leads to a slight distortion or tensile strain of the film especially in the rolling direction. In the case of filter structures for screens for three-dimensional display, which feature a periodicity, this will impair the accuracy of periodicity, which may make such a filter unserviceable. This is because the optical filter structures have to be precisely matched to the pixel spacing of the display screen—as a rule, a liquid crystal display (LCD).
The method of fabrication described above causes problems also if several of these optical filter structures are applied onto the substrate simultaneously. Smaller structures can be used, for example, for the display screens of mobile phones, navigation systems or portable game consoles. If several filter structures are applied, for example, onto a glass substrate, the different filter structures need to be separated; i.e. the common glass substrate has to be cut, together with all other layers, from the base film to the cold laminating sheet to the lower adhesive coating. This can be done by means of suitable carbide-tipped glass cutters, but time and again this causes unforeseeable, irregular break edges and chips so that the entire, already manufactured optical filter has to be scrapped. Sometimes, the system of layers as a whole prevents clean separation and also causes sharp-edged fractures that not only make the product unfit for further use but also involve a severe risk of injury. It is known that operators, even though they wear gloves, frequently get their fingers cut.
Printing the optical filter structures onto the glass surface as an alternative fabricating method is unsuccessful, as the filter structures will not properly adhere to the optically smooth glass surface. The screen printing process will not produce optical structures of the necessary smallness and accuracy either. Structure edges and corners will flow out due to the surface tension of the viscous glass inks before curing or heat-fusing.